The present invention relates to electrodes capable of detecting a gas in a fluid and more particularly to electrodes capable of measuring the presence and the amount of oxygen.
One of the analytical methods of oxygen determination is the amperometric method. This method is quite rapid, simple in operation and is especially suited for determining either gaseous or dissolved oxygen in liquids. In the amperometric method, an electrode such as that shown in FIG. 1 is employed. In such an electrode, generally indicated as 10, body 12 is provided having an enclosure 14 therein containing means 16 for supporting a gold cathode 18. A membrane holder 20 and cap 22 are provided to hold a polymeric membrane material 24 such as polytetrafluoroethylene of a thickness as from 0.00025 inches to 0.002 inches stretched over the gold cathode 18 thereby completely enclosing enclosure 14. Enclosure 14 is then filled with an electrolyte 26, typically a 5% KCl solution, either buffered or unbuffered. Additionally, an anode 28, typically silver, is disposed within enclosure 14 in contact with electrolyte 26. A potential of 750 millivolts is applied between the anode 28 and the cathode 18 by means 29 connected thereto. In stretching the membrane 24 over cathode 18, a very minimal amount of electrolyte 26 is contained between gold cathode 18 and membrane 24. As a sample fluid is brought in contact with membrane 24, oxygen diffuses through membrane 24 to contact the gold cathode 18 in the presence of the electrolyte 26. A current flow results which is linear with the partial pressure of oxygen being sampled. Thus, this current can be measured and correlated to the amount of oxygen in the sample.
While this method is satisfactory in many applications, it suffers drastically in atmospheres of high CO.sub.2 such as encountered in monitoring automobile exhaust or stack gas emissions. Such an electrode as that of FIG. 1, as described above, becomes less sensitive to O.sub.2 upon even brief exposure to high concentrations of CO.sub.2 and may take several hours to recover so as to indicate the proper value of O.sub.2. The response of such an electrode is highly dependent on the pH of the electrolyte at the interface between the gas diffusing through the membrane and the cathode. At low, or acid, pH levels the response is low. At high, or base, pH levels the response is higher. Typical responses by a prior art electrode are shown in FIG. 2 and FIG. 3. In FIG. 2, the electrode was first exposed to ambient air containing approximately 21% O.sub.2. It was then exposed to pure nitrogen. FIG. 2 shows the response which was both expected and achieved. Upon the exposure to N.sub.2, the response dropped to the zero line. Upon exposure to air, the response climbed to the level indicating approximately 21% O.sub.2. This cycle was repeatable without problem. Referring now to FIG. 3, the expected and actual response of a prior art electrode is shown when the electrode was exposed to ambient air and then exposed to a mixture of 15% CO.sub.2 plus 3% O.sub.2 and the balance nitrogen. When exposed to the moxture, the expected response is for the output to drop to the 3% level, being an indication of the 3% O.sub.2 content of the mixture. Upon exposure to ambient air, it is expected that the output will climb to the 21% oxygen level of the ambient air. The actual response, however, was not as anticipated. When the electrode was exposed to the mixture, the response fell to the expected 3% O.sub.2 level. When the electrode was subsequently exposed once again to the ambient air sample, the output overshot the 21% level, then reversed and undershot the 21% level, and then slowly approached the 21% level asymptotically. It was found that the recovery period required for the output to attain the actual 21% level varied depending both on the duration of exposure to carbon dioxide and the amount of carbon dioxide in the sample. For example, when using such a prior art electrode on an automobile exhaust, an exposure for a period one minute to the exhaust gases resulted in a recovery period on the order of two to three hours before an accurate ambient response could be attained.
This phenomenon is a result of the small volume of electrolyte trapped adjacent to the gold cathode by the porous membrane. This is typically on the order of 1 microliter. As previously mentioned, the response of such a cell is dependent on the pH of the electrolyte. When CO.sub.2 is introduced, carbonic acid is formed which, when mixed with such an extremely small volume of basic electrolyte, results in a change of the pH of the electrolyte adjacent the gold cathode. In the typical electrolyte having a pH of approximately 13.5, the introduction of carbonic acid having a pH in the order of 4.5 results in a change of pH of the electrolyte trapped adjacent the gold cathode to a level of approximately 9. The response of the electrode will be correspondingly reduced until such time as the pH can attain its normal value by diffusion of normal electrolyte into the space between the membrane. The initial overshoot observed is, presumably, caused by the sudden change in pH and the unsettling of the electrolyte in the cathode area.
Attempts at improving the performance of oxygen electrodes are not new in the art. It is well known that changes in pH of the electrolyte adjacent the cathode will change the response of the electrode. On the other hand, it is known that the thickness of the electrolyte in this same area affects the sensitivity of the electrode to oxygen. Thus, ideally, the spacing between the membrane and the cathode is kept minimal while means are provided for allowing the free movement of the electrolyte through the space. Thus, in the prior art, it has been suggested to roughen the surface of the gold cathode, provide channels therein for the movement of electrolyte, and depose porous materials between the membrane and the cathode to provide channels for the movement of the electrolyte.
Such prior art suggestions have resulted in oxygen electrodes of marginal sensitivity and poor response times for certain applications. In particular, automobile exhaust analysis and flue gas analysis provide environments imposing restrictions beyond the capabilities of prior art O.sub.2 electrodes employing such techniques. In the field of automobile exhaust gas analysis, the ability to cycle and recover at rapid rates is imperative in "assembly line" type testing environments.
Therefore, it is the object of the present invention to provide an oxygen electrode which is sensitive to the partial pressure of oxygen to a degree allowing it to be satisfactorily employed in critical applications such as automobile exhaust gas analysis and flue gas analysis while at the same time having a virtually instantaneous recovery rate following exposure to high levels of carbon dioxide concentration.